The homunculus argument is a fallacy arising most commonly in the theory of vision. One may explain (human) vision by noting that light from the outside world forms an image on the retinas in the eyes and something (or someone) in the brain looks at these images as if they are images on a movie screen (this theory of vision is sometimes termed the theory of the Cartesian Theater: it is most associated, nowadays, with the psychologist David Marr). The question arises as to the nature of this internal viewer. The assumption here is that there is a ‘little man’ or ‘homunculus‘ inside the brain ‘looking at’ the movie.

The reason why this is a fallacy may be understood by asking how the homunculus ‘sees’ the internal movie. The obvious answer is that there is another homunculus inside the first homunculus’s ‘head’ or ‘brain’ looking at this ‘movie’. But how does this homunculus see the ‘outside world’? In order to answer this, we are forced to posit another homunculus inside this other homunculus’s head and so forth. In other words, we are in a situation of infinite regress. The problem with the homunculus argument is that it tries to account for a phenomenon in terms of the very phenomenon that it is supposed to explain.

Suppose instead that the homunculus is outside the brain. Why, for example think about the experimenter doing research on your vision. The fallacy functions as well, because now we have another homunculus (outside the brain) who looks at the movie screen (i.e. the measurements he performed on your visual system, in the medium controlled by him). “But how does this homunculus see the ‘outside world’?” Infinite regression again.

If you think that is outrageous, then let me give you an example. The exterior homunculus (experimenter) explains your vision by interpreting the controlled space he put you in (the lab) and the measurements he performed. When he does this interpretation he relies on:

physical laws

geometrical assumptions

statistical assumptions

at least. Suppose that the experimenter says: “to the subject [i.e. you] was presented a red apple, at distance , at coordinates . By the physical laws of opticks and by the geometrical setting of the controlled lab we know that the sensor of the retina of the left eye was stimulated by the light coming from the apple. We recorded a pattern of activity in the regions of the brain, which we know from other knowledge (and statistical assumptions) that is busy with recognition of fruits, is involved in contour recognition and with memories from childhood.” I agree that is a completely bogus simplification of what the real eperimenter will say, but bear with me when I claim that the knowledge used by the experimenter for explaining how you see the apple has not much to do with the way you see and recognize the apple. In the course of the explanation, the experimenter used knowledge about the laws of optics, used measurements which are outside you, like coordinates and geometric settings in the lab, and even notions from the experimenter’s head, as “red”, “apple” and “contours”.

Should the experimenter not rely on physical laws? Or on geometrical asumptions (like the lab is in a piece of euclidean 3d space)? Of course he can rely on those. Because, in the case of physical laws, we recognize them as physical because they are invariant (i.e. change in a predictable way) on the observer. Because in the case of geometrical assumptions we recognize them as geometrical because they are invariant on the parametrization (which in the lab appears as the privilege of the observer).

But, as it is the case that optics can explain only what happens with the light until it hits the retina, not more, the assumptions in the head of the experimenter, even physical and geometrical, cannot be used as an explanation for the way you see. Because, simply put, it is much more likely that you don’t have a lab in the head which is in a euclidean space, with an apple, a lamp and rules for measuring distances and angles.

You may say that everybody knows that apples are not red, that’s a cheap shot because apples scatter light of all frequencies and it just happen that our sensors from the retina are more sensible at some frequencies than other. Obvious. However, it seems that not many recognize that contours are as immaterial as colors, they are in the mind, not in reality, as Koenderink writes in Theory of “Edge-Detection” JJ Koenderink – Analysis for Science, Engineering and Beyond, 2012.

The explanation of vision which uses an exterior homunculus becomes infinite regression unless we also explain how the exterior homunculus thinks about all these exterior facts as lab coordinates, rational deductions from laws of physics and so on. It is outrageous, but there is no other way.

Let’s forget about experiments on you and let’s think about experiments on fly vision. Any explanation of fly vision which uses knowledge which is not, somehow, embodied in the fly brain, falls into the (exterior) homunculus fallacy.

So what can be done, instead? Should we rely on magic, or say that no explanation is possible, because any explanation will be issued by an exterior homunculus? Of course not. When studying vision, nobody in the right mind doubts about the laws of optics. They are science (i.e. reproducible and falsifiable). But they don’t explain all the vision, only the first, physical step. Likewise, we should strive for giving explanations of vision which are scientific, but which do not make appeal to a ghost, in the machine or outside the machine.

5 thoughts on “On the exterior homunculus fallacy”

Many people, when realizing that the image is turned upside down when projected on the retina, think that it has to be a way by which the brain turns the image upright. I heard this even from computer programmers working in 3D, and even from physicists (http://physics.stackexchange.com/questions/62481/why-we-see-upright-images/). I believe this thinking relies implicitly on the homunculus fallacy.